(1) Field of the Invention
The present invention relates to an image forming apparatus, particularly relating to the improvement of the sheet conveying system in an image forming apparatus such as a copier, printer, PPC facsimile etc.
(2) Description of the Prior Art
FIG. 1 is a sectional view showing a copier typically used as an image forming apparatus in the field of the industry. In the figure, an original document is placed on a table glass 2 in the upper part of a copier machine body 1. As the copy start button (not shown) is operated, an exposure lamp 3 illuminates the original and the reflected light having the image information is passed through first through sixth mirrors 4 to 9 and a lens 24 to a photoreceptor 10. To scan the entire document, the exposure lamp 3 as well as the first to third mirrors (4 to 6) is driven via unillustrated wire by means of a scanner motor 23. The photoreceptor is electrified by a main charger 11. The illuminated image on the photoreceptor is developed in a developing unit 12. A sheet is fed from a cassette 21 and is delivered out by a feed roller 17. The developed image on the photoreceptor is transferred to the delivered sheet by a transfer charger 13 and the sheet is separated from the photoreceptor by a separating charger 14 and is conveyed by a suction belt 18 to a fixing roller 19 where the transferred image on the sheet is fixed. The finished copy is discharged to a sheet output unit 20. Reference numerals 15 and 16 designates a cleaning unit and an charge-erasing charger, respectively. Here, the photoreceptor 10, the roller in the developing unit 12, the feed roller 17, the suction belt 18, the fixing roller 19 are driven by driving forces which are transmitted through belts, gears etc., from an unillustrated main motor. Reference numerals 52 through S7 in FIG. 1 indicate sheet detecting sensor arranged in the sheet conveyance path and S1 indicates a sheet-detecting sensor arranged so as to detect the emptiness of the cassette 21.
By the way, the sheet feeding portion as well as the sheet conveying portion in the conventional image forming apparatus thus configured has used either of two kinds of sensors, specifically, the contact-type sensor shown in FIGS. 2A and 2B or the reflection-type sensor shown in FIG. 3, as the aforementioned detecting sensors for detecting the sheet. FIG. 2A is a perspective view showing an example of the contact-type sensor, and FIG. 2B is a side view of FIG. 2A. In the figure, 30 designates a contact-type sensor body and 31 designates an actuator as a detecting lever. The actuator 31 is mounted on a pivot 32 and urged by a spring 33 so that the free end of the actuator 31 will press an object (not shown) to be detected. One end of the spring is engaged on a spring stopper surface 35 while the base of the actuator 31 is stopped by an actuator stopper surface 34 so as to prevent an excessive rotation. The rotation of the actuator 31 when the object to be detected is sensed is outputted as an electric signal by one of publicly known electrical or mechanical means, whereby the detected signal is supplied to the control system. FIG. 3 is a perspective view showing an example of the reflection-type sensor. In the figure, 40 designates a reflection-type sensor body and 41 designates a light emitting/receiving section. For the structure of the reflection-type sensor and the output scheme of the signal, any of publicly known methods can be applied appropriately.
On the other hand, Japanese Patent Application Laid-Open Sho 60 No. 216,359 discloses a system in which, by using a pair of sensors, namely a contact-type sheet sensor and a reflection-type sensor disposed in the manual bypass feeding section or the sheet feeding cassette, it is distinguished whether the sheet to be fed is a plain paper sheet or an OHP film sheet based on the detected outputs from the two sensors, and in accordance with the determined result, the processing conditions for image forming (in this disclosure, it is referred that the fixing speed and the generation of the transfer high-voltage current are controlled in accordance with the type of the sheet) are set up.
Further, Japanese Patent Application Laid-Open Sho 64 No. 82,049 discloses a system in which, by using a pair of sensors, namely, a contact-type sheet sensor and a reflection-type sensor disposed along the sheet conveyance path on the upstream of the transfer station, it is distinguished whether the conveyed sheet is a plain paper sheet or an OHP film sheet based on the detected outputs from the two sensors, and in accordance with the determined result, the transfer charger output and the fixing speed are set up in accordance with the type of the sheet.
However, in the above documents which are publicly known, the fed sheet is distinguished at only the feeding port by the contact-type and reflection-type sensors provided therein, and all the sheet detecting sensors do not use both the contact-type and reflection-type sensors.
That is, the conventional systems use any of the contact-type and reflection-type sensors to detect sheets. In this case, the reflection-type sensor can not perform detection of a sheet if the sheet is of OHP film. The contact-type sensor would have detecting failures if sheets are successively fed at shorter intervals since the output signal from the sensor is caused to chatter due to the movement of the lever. For example, as shown in FIG. 4A, if the output signal from the contact-type sensor is chattering, the detection of the sheet during chattering should be ignored. However, if the interval between one sheet and the next becomes shorter, the signal behaves as seen in FIG. 4B, making it impossible to detect the sheets. Here, FIG. 4A is a timing chart representing the chattering of the contact-type sensor when sheets are fed at sufficiently long intervals; and FIG. 4B is a timing chart showing the chattering of the same sensor when sheets are fed at shorter intervals. In theses charts, the vertical axis represents ON/OFF states, and the horizontal axis (t-direction) indicates the timing (of the passage time of the sheet).
Thus, in order to feed plain paper sheets at a high rate (or at closer intervals between one sheet to the next), the reflection-type sensor which is free from chattering will have to be used since the contact-type sensor has a limit to shorten the feeding interval- However, the use of only the reflection-type sensor cannot deal with the detection of OHP sheets. Therefore, there was no method which was able to both feed plain paper sheets at a high rate and perform, by any means, a proper copying operation for the OHP sheet.
Further, in accordance with the above two publicly known techniques, the pair of the sensors (contact-type and reflection-type sensors) provided in the sheet feeding port (portion) is used to distinguish between plain paper sheets and OHP sheets in order to set up the charger output in the transfer station and the fixing speed based on the detected result. However, the two sensors are arranged only at the sheet feeding port (portion), while only one kind of sensor is arranged at any other locations along the sheet conveyance path, respectively.
Anyway, in accordance with any of the conventional methods, as to conveying a plain paper or OHP film sheet (from the feeding port to the output port), there is no technical idea to effectively achieve the sheet conveyance in order to improve the processing performance of the image forming apparatus, by optimizing the sheet conveyance speed.